Lecithin cholesteryl acyltransferase (LCAT) is a 63 Kd enzyme that is responsible for the esterification of virtually all free cholesterol in plasma and thus, is essential for the process of reverse cholesterol transport. Cholesteryl esters formed by LCAT are preferentially incorporated into HDL particles. Patients with functional defects in LCAT can present with remarkably different clinical features, based on either a partial enzyme deficiency, designated Fish Eye Disease (FED) or a total enzyme deficiency, designated Classical LCAT Deficiency (CLD). In our studies we have investigated the underlying molecular defects in the LCAT gene that lead to the expression of these two phenotypically distinct clinical syndromes. We have identified a novel mutation that leads to FED in a patient of French descent presenting with corneal opacifications and HDL-cholesterol levels of less that 10 mg/dl but no renal disease. DNA sequencing and RFLP analysis revealed that the patient was a true homozygote for a C to T mutation resulting in the substitution of Arg-158 by Cys. In vitro expression and characterization of the variant demonstrated that the mutant enzyme had 35% of the specific alpha-LCAT activity of normal LCAT. Using nature lipoprotein substrates, LCAT-158 was able to esterify LDL but not HDL, indicating that the 158 mutation does not disrupt LCAT catalytic function but affects the ability of the mutant enzyme to interact and esterify free cholesterol present in HDL. In addition, despite significant reduction in the patient's plasma cholesterol esterification rate (to less than 20 percent) the clinical complications of renal disease and anemia were not evident in the patient indicating that this level of LCAT function is sufficient to prevent these clinical manifestations. Our studies provide new insights into the structure- function relationship of LCAT as well as into the clinical heterogeneity observed in patients with LCAT gene defects.